Admit it, you need your coffee to be that perfect shade of caramel or you'll jinx the whole damn day. Whether you're the touch-of-cream or leave-lots-of-room type, the perfectly lightened coffee is a key part of your finely tuned morning routine (unless you drink it black, which smartly reduces the number of variables that can mess up your morning).
Perhaps you've mastered the perfect shade of hot java. But when it comes to iced coffee, you may have noticed, your little pour of cream creates a much lighter-coloured cup than you expect.
If you blame this phenomenon on the fact that iced coffee is diluted by ice, you'd be mostly wrong. Sure, ice dilution influences the colour of your coffee, but that happens mostly in crappy coffee shops where they don't refrigerate the brew in advance to preserve its strength.
In finer establishments, the coffee is already cold when the ice is added, and the brew masters will often make the coffee a bit stronger to compensate for melting ice. And even then, iced coffee will lighten in colour much more quickly than hot coffee.
Mike Sakillaris, a chemical engineer who specialises in fluid mechanics at Dow, explained why. Milk and half-and-half, he said, are emulsions of proteins. The emulsion is a blend of pure cream, the stuff that rises to the top when raw milk is left to its own devices, and skim milk, the stuff that stays on the bottom. Homogenisation brings them together in a process in which the two substances are pumped at very high pressure through a thin tube onto a plate. This blasts the fat molecules apart allowing them to stick to water molecules naturally present in the skim milk. An emulsion is created.
The emulsion is what gives milk its distinctive white colour. When the dairy hits hot coffee, the heat breaks the emulsion into simple proteins and water, which also muddies the white colour.
In iced coffee there's no heat to break up the emulsion, so the milk or cream's white colour remains intact, lightening the coffee more efficiently.
How does that influence the coffee's taste? Good question, Sakillaris said. Taste is too subjective to base an answer on science alone.
"If you break the emulsion, there's got to be some effect on taste, but it's difficult to quantify," he said.
So we did our own taste test. Sakillaris suggested tasting half-and-half alone to eliminate the influence of the coffee's flavour. If the heated milk tastes different on its own, we can assume it will also taste different when added to coffee. So we heated half-and-half on a gas stove for one minute to about 60C, and compared it to the same half-and-half straight from an approximately 2C refrigerator. The cold version had a slightly sour tang that brought to mind plain yogurt. That aspect was missing from the the warm version, which we found to have a more mellow flavor.
So take this into consideration next time you lighten your cup of Joe. You'll need less milk or half-and-half for iced coffee, and don't be surprised if it tastes slightly tangier than hot java.